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Message-ID: <20230412072555.38c7288f@kernel.org> Date: Wed, 12 Apr 2023 07:25:55 -0700 From: Jakub Kicinski <kuba@...nel.org> To: Hangbin Liu <liuhangbin@...il.com> Cc: Jay Vosburgh <jay.vosburgh@...onical.com>, netdev@...r.kernel.org, "David S . Miller" <davem@...emloft.net>, Jonathan Toppins <jtoppins@...hat.com>, Paolo Abeni <pabeni@...hat.com>, Eric Dumazet <edumazet@...gle.com>, Liang Li <liali@...hat.com>, Simon Horman <simon.horman@...igine.com>, Miroslav Lichvar <mlichvar@...hat.com> Subject: Re: [PATCHv3 net-next] bonding: add software tx timestamping support On Wed, 12 Apr 2023 20:28:08 +0800 Hangbin Liu wrote: > > Ok, maybe I didn't look at that carefully enough, and now that I > > do, it's really complicated. > > > > Going through it, I think the call path that's relevant is > > taprio_change -> taprio_parse_clockid -> ethtool_ops->get_ts_info. > > taprio_change is Qdisc_ops.change function, and tc_modify_qdisc should > > come in with RTNL held. > > > > If I'm reading cscope right, the other possible caller of > > Qdisc_ops.change is fifo_set_limit, and it looks like that function is > > only called by functions that are themselves Qdisc_ops.change functions > > (red_change -> __red_change, sfb_change, tbf_change) or Qdisc_ops.init > > functions (red_init -> __red_change, sfb_init, tbf_init). > > > > There's also a qdisc_create_dflt -> Qdisc_ops.init call path, > > but I don't know if literally all calls to qdisc_create_dflt hold RTNL. > > > > There's a lot of them, and I'm not sure how many of those could > > ever end up calling into taprio_change (if, say, a taprio qdisc is > > attached within another qdisc). > > > > qdisc_create also calls Qdisc_ops.init, but that one seems to > > clearly expect to enter with RTNL. > > > > Any tc expert able to state for sure whether it's possible to > > get into any of the above without RTNL? I suspect it isn't, but I'm not > > 100% sure either. > > You dug more than me. Maybe we can add an ASSERT_RTNL() checking here first? > But since we can't 100% sure we are holding the rtnl lock, I think we > can keep the rcu lock for safe. I saw rlb_next_rx_slave() also did the same... ASSERT_RTNL sounds good. I think that drivers may expect rtnl lock to be held around ethtool ops, so if some path is not holding it - I'd count that as a bug. > > >You could check in this loop if TX is supported... > > > > I see your point below about not wanting to create > > SOFT_TIMESTAMPING_SOFTRXTX, but doesn't the logic need to test all three > > of the flags _TX_SOFTWARE, _RX_SOFTWARE, and _SOFTWARE? > > I think Jakub means we have already add _RX_SOFTWARE and _SOFTWARE for bonding > whatever slave's flag, then we just need to check slave's _TX_SOFTWARE flag. Indeed.
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